Devices and methods for heat bending glass sheets are well known in the art. For example, see U.S. Pat. Nos. 5,383,990; 6,240,746; 6,321,570; 6,318,125; 6,158,247; 6,009,726; 4,364,766; and 5,443,669.
FIG. 1 is a schematic diagram illustrating a conventional apparatus and method for heat bending glass sheets in making a laminated product such as a vehicle windshield. Vehicle windshields are typically curved, and thus require first and second curved (as a result of heat bending) glass sheets laminated to one another via a polymer interlayer. First glass substrate 1 has a multi-layer solar control coating 3 thereon (e.g., low-E coating including at least one IR reflecting layer of a material such as Ag); while second glass substrate 5 is not coated.
Referring to FIG. 1, two flat glass substrates 1, 5 are placed in a bending furnace (e.g., on a bending mold) in an overlapping manner by interposing an optional lubricating powder (not shown) such as sodium hydrogen carbonate, cerite, magnesium oxide, silica, or the like between contacting surfaces of the two glass substrates. The glass substrates 1, 5 are then heated using infrared (IR) emitting heating elements 7 to a processing temperature(s) near a softening point of the glass (e.g., from about 550 to 850 degrees C., more preferably from about 580 to 750 degrees C.) in order to soften the overlapping glass substrates 1, 5. Upon softening, the glass substrates 1, 5 (including any solar control coating 3 thereon) are bent by their deadweight (i.e., sagging) along a shaping surface of a bending mold (not shown) into the desired curved shape appropriate for the vehicle windshield being made. A press bending apparatus may optionally be used after the glass is sufficiently softened (the press bending may be conducted as the final step before cooling the glass).
After being heat bent in such a manner, the bent glass substrates 1, 5 (with solar control coating 3 still on substrate 1) are separated from one another and a polymer inclusive interlayer sheet (e.g., PVB) is interposed therebetween. The glass substrates 1, 5 are then laminated to one another via the polymer inclusive interlayer 9 in order to form the resulting vehicle windshield shown in FIG. 2.
Different vehicle windshield models require different shapes. Some shapes require more extensive bending than others. As windshields requiring extensive bending are becoming more popular, the need for high performance solar control coatings (e.g., including one or more IR reflecting Ag layers) has also increased. An example high performance solar control coating 3 is disclosed in WO 02/04375 (and thus counterpart U.S. Ser. No. 09/794,224, filed Feb. 28, 2001), both hereby incorporated herein by reference.
Unfortunately, it has been found that when using conventional glass bending techniques, certain solar control coatings cannot on a regular basis withstand the bending process(es) sometimes used. Set forth below is an explanation as to why certain solar control coatings have a hard time withstanding conventional heat bending processes without suffering undesirable damage such as reduced transmission.
Conventional glass bending heating elements emit IR radiation 8 in the near, mid and far IR ranges. By this we mean that heating elements 7 emit each of near-IR (e.g., 700–4,000 nm; or 0.7 to 4.0 μm), mid-IR (4,000–8,000 nm; or 4–8 μm), and far-IR (>8,000 nm; or >8 μm) radiation. In certain instances, the near-IR range may be considered from 0.7 to 3.0 μm and the mid-IR range from 3–8 μm. Herein, IR radiation is defined as wavelengths of 0.7 μm and above with known constraints.
Each of these different types (i.e., wavelengths) of IR radiation impinges upon the glass substrates 1, 5 to be heated and bent. Certain IR radiant heaters work in a manner such that turning up the power for the same results in significantly more near-IR radiation being emitted. In any event, much of the IR radiation from conventional heaters that reaches the glass to be bent is in the near-IR range, as the peak of this IR radiation is often in the near-IR range. In certain example instances, at least about 50% of the IR radiation that reaches the glass to be bent is in the near-IR range, sometimes 70% or higher. For instance, a heater with black body properties operating at 538 degrees C. emits 32.8% of its energy from 0.7 to 4 μm, 44.7% from 4 to 8 μm and 22.5% in wavelengths greater than 8 μm A heater with black body properties operating at 871 degrees C. emits 57.6% of its energy from 0.7 to 4 μm, 31.9% from 4 to 8 μm and 10.5% in wavelengths greater than 8 μm. A heater with black body properties operating at 1,094 degrees C. emits 68.7% of its energy from 0.7 to 4 μm, 24.4% from 4 to 8 μm and 6.9% in wavelengths greater than 8 μm. The total power emitted increases with temperature proportional to the absolute temperature raised to the fourth power. For the three temperatures listed above, the total emitted power is approximately 15, 63 and 125 watts/inch square, respectively. The power for 0.7 to 4 μm is 4.9, 36.3, and 85.9 watts/inch square, respectively.
U.S. Pat. No. 6,160,957 discloses a heating element including a resistor element mounted on a ceramic fiber material such as aluminosilicate in spaced relation thereto. However, in the '957 Patent it is the resistor element (not the ceramic fiber) which emits the IR radiation toward the product to be heated.
As shown in FIG. 3, it has been found that typical soda lime silica glass (often used for substrates 1, 5) has a high absorption of IR radiation in the mid-IR and far-IR ranges. In other words, soda lime silica glass absorbs much of incident IR radiation at wavelengths above about 3–4 μm (microns). Thus, in the mid and far-IR ranges, the glass absorbs much of the IR radiation before it can reach the coating. It is believed that this absorption in the mid and far-IR ranges is due to at least water, Si—O and Si—O—H absorption in the glass matrix. FIG. 3 shows that soda lime silica glass is substantially opaque to IR radiation above 3–4 μm, but rather transmissive of IR radiation below 3–4 μm. Unfortunately, the transmissive nature of the glass at wavelengths less than 3–4 μm means that a significant amount of IR radiation in the near-IR range (from 0.7 to 3–4 μm) is not absorbed by the glass substrate(s) 1 and/or 5 and as a result passes therethrough and reaches solar control coating 3. As used herein, the phrase “from 0.7 to 3–4 μm” means from 0.7 μm to 3 and/or 4 μm. The amount of energy in this wavelength band (watts/inch2) increases as the temperature of the elements increases. Typically, the power applied in later times in the bending process is substantially higher than earlier times so that the amount of energy not absorbed by the glass and thus by the coating increases as the bending process proceeds.
Unfortunately, certain of this near-IR radiation which is not absorbed by the glass substrate and thus reaches solar control coating 3, is absorbed by the coating 3 (e.g., by Ag layer(s) of the coating) thereby causing the coating 3 to heat up. This problem (significant heating of the coating) is compounded by: (a) certain solar control coatings 3 have a room temperature absorption peak (e.g., of 20–30% or more) at wavelengths of approximately 1 μm in the near IR range, at which wavelengths the underlying glass is substantially transmissive, and (b) the absorption of many solar control coatings 3 increases with a rise in temperature thereof (e.g., sheet resistance Rs of Ag layer(s) increase along with rises in temperature). In view of (a) and (b) above, it can be seen that the peak absorption of certain solar control coatings 3 at near-IR wavelengths of about 1 μm can increase from the 20–30% range to the 40–60% range or higher when the coating temperature increases from room temperature to 500 degrees C. or higher, thereby enabling the coating to heat up very quickly when exposed to significant amounts of near-IR wavelengths. The temperature of the coating may be mitigated by conduction of the absorbed energy into the bulk glass, but the rate of this process is finite. If energy is applied to the coating faster than it can be dissipated into the bulk, a thermal gradient is created leading to substantial overheating of the coating which leads to coating damage. The potential for coating overheating is often highest in the later stages of the bending process when the glass and coating are near the softening point, e.g., due to the higher amounts of near-IR heat being generated by the heating element(s) and due to absorption of the coating being higher.
Coating 3 is more susceptible to being damaged when it is unnecessarily heated up during the glass bending process. When coating 3 is damaged (e.g., visible transmittance drops significantly), the bent glass substrate 1 with the damaged coating thereon is typically discarded and cannot be commercially used.
This problem (i.e., coating overheating) also affects the shapes that can be attained in the bending process. If heat is applied only from one side (e.g., from the top in FIG. 1), the presence of the coating on substrate 1 versus substrate 5 limits the radiant energy that can be absorbed by the substrate 5; so the substrate 1 may become more soft than substrate 5. This means that substrate 1 must often be overheated in order to enable substrate 5 to reach a desired temperature for sagging to a desired shape. Application of heat to both the top and bottom (see FIG. 1) provides radiant heat directly to both substrates, but also causes the coating to receive double the amount of energy potentially leading to overheating.
It can be seen that certain solar control coatings 3 have a narrow thermal stability range that can limit the shape (i.e., degree of bending) of glass attainable in a bending process. Highly bent windshields often require higher bending temperatures and/or long bending times which certain coatings 3 cannot withstand given conventional glass bending techniques.
An object of this invention is to minimize the time at and/or peak temperature attained by a solar control coating 3 during a heat bending process for bending and/or tempering a glass substrate that supports the coating.
Another object of this invention is to provide an apparatus and/or method for heat bending and/or tempering glass substrates/sheets, designed to reduce the amount of near-IR radiation that reaches the glass substrate(s) to be bent.
Another object of this invention is to provide a filter (or baffle) for filtering out at least some near-IR radiation before it reaches a glass substrate to be bent and/or tempered. This can enable a solar control coating supported by the glass substrate to reach a lesser temperature than if the filter was not provided.
By enabling the maximum coating temperature to be reduced (and/or the time at which the coating is at a maximum temperature to be lessened), certain embodiments of this invention can realize one or more of the following advantages: (a) the solar control coating is less likely to be damaged during the bending and/or tempering process of an underlying glass substrate, (b) higher degrees of bending of an underlying glass substrate can be achieved without damaging the solar control coating; (c) heating time and/or maximum coating temperature can be reduced without reducing the amount of glass bend, and/or (d) power consumption of the heater may be reduced in certain instances.
In certain example embodiments of this invention, a filter (e.g., baffle or the like) of or including a ceramic (e.g., a silicate such as aluminosilicate) is used which reduces the amount near-IR radiation which reaches the glass substrate and/or coating to be bent and/or tempered.
Another object of this invention is to fulfill one or more of the above-listed objects.
In certain example embodiments of this invention, one or more of the above-listed objects is/are fulfilled by providing an apparatus for bending and/or tempering a glass substrate, the apparatus comprising: a heating element for generating energy; and a near-IR filter comprising a ceramic radiating surface located between the heating element and the glass substrate, the near-IR filter for reducing the amount of near-IR radiation that reaches the glass substrate to be bent and/or tempered.
In other example embodiments of this invention, one or more of the above-listed objects is/are fulfilled by providing a method of bending glass, the method comprising: providing a glass substrate having a solar control coating thereon; directing IR radiation at the glass substrate from a heating layer comprising ceramic in order to heat the glass substrate to a temperature of at least about 550 degrees C. for bending; and wherein less than about 30% of the IR radiation reaching the glass substrate is at wavelengths from 0.7 to 3.0 μm.